OBJECTIVES: After crushing the motor nerve to one cutaneous pectoris muscle of the frog, it regenerates and innervates the denervated muscle. The undamaged nerve to the contralateral muscle also responds; it sprouts and forms additional synaptic connections with already innervated muscle fibers (hyperinnervation). My specific objectives in the proposed study are: 1) study the factors involved in the induction of sprouting and hyperinnervation; 2) examine functional and structural aspects of synapse formation in innervated muscle fibers during hyperinnervation; 3) determine the changes that occur in somata of sprouting motor neurons and their central connections in the spinal cord. METHODOLOGY: The preparation to be used is the cutaneous pectoris nerve and muscle of the frog. In this preparation, the same synapses studied physiologically can be examined by a variety of histological techniques. Furthermore, cell bodies of motor neurons that innervate specific muscles can be identified and studied. SIGNIFICANCE: The understanding of sprouting and synapse formation are basic for our knowledge of how specific connections between neurons and their target cells are established, how they are maintained, how they may be altered and how they are replaced after damage. Preliminary results suggest a new type of interaction between nerve cells, where the signal for sprouting may be communicated in the spinal cord from injured nerve cells to intact motor neurons.